It is difficult to gauge how difficult porting will be until the job is done, & having prior experience certainly helps.

In general, there are two central issues which have to be resolved in porting any application to OpenBSD:

Is the application written in a POSIX-compliant manner? Or was the intent to specifically target Linux? Does it use libraries which are not present on OpenBSD?

All of these questions can potentially mean that you may need to spend significant time researching & porting supporting libraries required by the application.

Secondly, do assumptions made by the application conform to the filesystem layout used by OpenBSD (see the manpage to hier(7) for more information...)? Making an application found in the wild conform to OpenBSD's layout may take even more time & thought.

These are the issues that have to be dealt with in porting any application to OpenBSD's environment. If you are up to the challenge of porting filezilla, go for it, but be prepared to put a lot of time to complete the task.

However, the simplest solution is to upgrade to a newer version of OpenBSD. According to Section 5.1 of the FAQ:

...any interest in support of OpenBSD 4.2 ended with the release of OpenBSD 4.4. You will not get help in porting either here or on the OpenBSD mailing lists. As of the first of November, OpenBSD 4.2 is no longer a supported version.

Again, the simplest solution is to upgrade versions of the operating system.